Zeolitic materials, both natural and synthetic, are known to have the capability for catalyzing various types of hydrocarbon conversion reactions which take place in the presence of catalytic sites with acidic functionality. These zeolite materials generally have ordered, porous crystalline structures within which there are a number of small cavities that are interconnected by a number of still smaller channels. These cavities and channels are precisely uniform in size within a specific zeolitic material. Since the dimension of these pores are such as to accept for adsorption purposes molecules of certain dimensions, while rejecting those of larger dimension, these materials have commonly been known to be "molecular sieves" and are utilized in a variety of ways to take advantage of the adsorptive propertis of these compositions. The structures may be determined by X-ray diffraction techniques.
These molecular sieves include a wide variety of positive ion-containing crystalline aluminosilicates and other siliceous materials such as borosilicates, ferrosilicates and gallosilicates in which the presence of the trivalent metal at sites within the silicate structure provides the desired acidic functionality in an environment which permits access to the site only by molecules of appropriate size so that the acid catalyzed reactions are carried out in a "shape selective" manner. Metal cations such as sodium which are usually present in these materials when they are synthesized may be converted to the hydrogen form by exchange with ammonium ions followed by heating to drive off ammonia or by direct exchange with an acid such as hydrochloric acid, if the zeolite is not degraded by the acid. Useful catalysts are also produced by a combination of ion-exchange treatments in which the crystalline silicate may be converted to the acid form and they may be ion exchanged with a solution of various metal salts to produce the metal exchanged zeolite.
The acid activity of aluminosilicate zeolites may be so high that conventional hydrocarbon conversion processes and apparatus cannot take full advantage of this high activity. For example, in catalytic cracking, high activity may yield excessive coke formation and the production of large amounts of light gases. The acid activity of zeolite catalysts may, however, be lowered to a level at which the use of such catalysts in catalytic conversions is satisfactory and, in fact, results in a considerable increase in the efficiency of such processes. Reactions which have been performed successfully over low acidity zeolites include the conversion of oxygenates such as methanol and dimethyl ether to olefin and other hydrocarbon products, xylene isomerization, aromatic alkylation, and olefin oligomerization, as well as catalytic dewaxing and hydrodewaxing and hydroisomerization.
One method of reducing the activity of aluminosilicate zeolite catalyst is by compositing the zeolite with a matrix material which is relatively inactive. Suitable matrix materials include inorganic oxides, such as those of silica, zirconia, alumina, magnesia and combinations of such materials with one another, as well as clays and other refractory materials.
Other methods to reduce the activity of acid zeolites include cation exchange with sodium or other alkali metal cations or by forming the zeolites with high silica:alumina mole ratios in the structure or framework. An important method in reducing the activity of zeolite catalysts is by a process of steam treating. By controlled steaming, it is possible to produce zeolite catalysts having any desired degree of activity. The degree of steaming of a specified catalyst to achieve a desired activity level is largely dependent upon the nature of the zeolite. Steam treatment, however, often requires long periods of time to treat the catalyst effectively for activity reduction.
U.S. Pat. No. 3,939,058 discloses methods of modifying the catalytic properties of zeolite cracking catalysts. One such method is calcination which is defined as heating at high temperatures but below the sintering temperature of the zeolite for varying periods of time. Other methods are also disclosed, including compositing the zeolite in a matrix and steam treatment. The patent further discloses that the crystallinity retention of catalysts may be improved by precalcination of the crystalline aluminosilicate. For example, the patent states that it has been found possible to preserve the crystallinity of aluminosilicates such as the rare earth exchange synthetic faujasites, by calcining the zeolite to drive off water, thus forming a more suitable structure and minimizing loss in crystallinity during subsequent rapid drying, as in spray drying, wet processing, steaming and aging. The calcining may be accomplished by heating the crystalline aluminosilicate sieve after ion exchange to a temperature below the sintering temperature of the sieve and generally in the range of from 500.degree. to 1600.degree. F. (about 260.degree. to 870.degree. C.).
Similarly, U.S. Pat. No. 4,141,859 discloses a method of controlling the relative acid activity of zeolite catalysts, by treating the zeolitic component with air or steam at elevated temperatures, e.g., up to 1700.degree. F. (about 925.degree. C.) in air or at temperatures from about 800.degree. F. to about 1700.degree. F. (about 425.degree. C. to 925.degree. C.) in steam.
Calcination of the freshly synthesized zeolite to remove adsorbed water and organic materials that have been used to form the zeolite crystals is necessary to activate the zeolite and accordingly has generally been employed. Also, as stated above, precalcination of the zeolite has been found to stabilize the crystallinity of the zeolite. However, heat treatment may remove hydroxyl groups from the framework of the zeolite. Thus, dehydroxylation of a decationized Y zeolite is discussed in Zeolite Chemistry and Catalysts, ACS Monograph 171, pages 142 and 143, in which dehydroxylation of Y zeolite is stated to result from prolonged calcination at relatively high temperatures, resulting finally in the structural collapse of the zeolite and the formation of an amorphous silica or silica-alumina structure. For these reasons, the use of high temperatures has generally been avoided in zeolite synthesis. When organic materials are to be removed from the freshly synthesized zeolite, temperatures of about 1000.degree. F. (about 540.degree. C.) are typical and generally not exceeded in order to avoid damage to the crystal structure.
Calcination or high temperature treatment has been employed in various catalyst treatments to achieve particular results, for example, to convert impregnated metal or other compounds to different forms as described in U.S. Pat. Nos. 4,276,438 and 4,060,568 or to destroy ion exchange capacity as described in U.S. Pat. No. 3,097,115. However, even in such cases the use of higher temperatures, e.g. above 500.degree. C., has not been preferred because of the undesirable effect on the structure of the zeolite.